Diffusion transfer processes are well known in the industry, and their details will not be described herein. Such processes are disclosed in detail for example in Photographic Silver Halide Diffusion Processes (A. Rott and E. Weyde, Focal Press, 1972), Imaging Processes and Materials, Neblette's Eighth Edition (J. Sturge, V. Walworth and A. Shepp, Van Nostrand Reinhold, 1989, Chapter 6, Instant Photography and Related Reprographic Processes), and Modern Photographic Processing, Vol. 2, G. Haist, John Wiley and Sons, 1979, Chapter 8, "Diffusion Transfer"). Various types of photographic materials can be prepared by diffusion transfer processes, and such preparation procedures are described in detail in the aforementioned sources. For example, it is known that a transfer image can be obtained when a light-sensitive element prepared by coating a silver halide emulsion on a support is superimposed on an image-receiving element prepared by coating an image-receiving layer containing silver precipitation nuclei on another support, and a treating element comprising a high viscosity alkaline treatment composition containing a developing agent and a silver halide solvent is spread between these two elements.
In the just described construction, a transfer image is obtained on the image-receiving element by subjecting the light-sensitive element to exposure, superimposing the exposed element on the image-receiving element, spreading the treating element between these two elements and then separating the image-receiving element and the exposed light-sensitive element from each other after a set period of time. Continuous efforts have been directed toward rapid completion of the transfer image.
As a means to quicken completion of the transfer image, a process has been proposed in which a highly reductive compound such as a hydroquinone compound is used as a developing agent in the treating element and a quickly soluble compound such as hypo is used as a silver halide solvent. In the case of this process, however, the transfer images obtained are very unstable and cannot be stored for prolonged periods of time because of generation of stains due to an oxidized product of the developing agent, sulfurization of residual hypo and the like. To prevent such problems, an anti-oxidation layer, such as a polyvinyl alcohol layer containing an alkali neutralizer, must be applied to the image surface immediately after the image is formed. The application of such anti-oxidation layers entails complex handling.
Another means to quicken completion of transfer images is a process in which highly soluble silver chloride, silver bromochloride or the like is used as the silver halide emulsion in the light-sensitive element. This process, however, has disadvantages. The low sensitivity of the light-sensitive element makes it impossible to apply this process to photographing purposes. Further, the nature of the light-sensitive elements used in this process is such that they are easily fogged and this causes a decrease in the density of the resulting transfer images.
JP-A-2-51155 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses that a pitch-black image with hard toe gradation can be obtained by the combined use of a silver halide emulsion containing 1 mol % or more of silver iodide, a hydroxylamine developer and a water soluble iodide. This process, however, is not preferable, because completion of images becomes slow and the hue becomes bluish when treated with the treating element containing a water-soluble iodide and in a silver iodide content in the silver halide emulsion of 5 mol % or more.
Research Disclosure (RD) No. 9245 proposes a developing solution for surface and internal use which contains 3-pyrazolidone, ascorbic acid, a nitrogen-substituted heterocyclic thione or thiol compound and an alkali metal iodide, and discloses that a high density image can be obtained with a high sensitivity by use of this developing solution. However, when this developing solution is applied to the diffusion transfer system of the present invention, a high density transfer image cannot be obtained quickly because .of insufficient solubility of the developing solution against the silver halide emulsion.